1. Field of the Invention
This invention relates to an FM detector, a signal interpolation method, and a computer program for the FM detector or the signal interpolation method.
2. Description of the Related Art
Japanese examined patent application publication number 6-71210 discloses a device for eliminating pulse noise in an FM receiver. In the device of Japanese publication number 6-71210, a front end converts an RF signal into an IF signal fed to an FM detection circuit and an AM detection circuit. The FM detection circuit is followed by an HPF-containing circuit for sensing the leading edge of every pulse noise in an output signal from the FM detection circuit, that is, the leading edge of every FM or PM pulse noise. The AM detection circuit is followed by an HPF-containing circuit for sensing the leading edge of every pulse noise in an output signal from the AM detection circuit, that is, the leading edge of every AM pulse noise.
In the device of Japanese publication number 6-71210, a gate circuit is connected between the FM detection circuit and a stereo demodulation circuit to selectively allow and block the transmission of the output signal of the FM detection circuit to the stereo demodulation circuit. A level hold circuit is provided between the gate circuit and the stereo demodulation circuit. In the event that the leading edge of an FM, PM, or AM pulse noise is sensed, the gate circuit is closed for a fixed time during which the level hold circuit continues to feed the stereo demodulation circuit with the output signal of the FM detection circuit which occurs immediately before the gate circuit is closed instead of the current output signal of the FM detection circuit. Thus, signal interpolation is implemented with respect to the output signal of the FM detection circuit while the gate circuit is closed. Specifically, the gate circuit is driven by a prescribed-width pulse outputted from a one-shot multivibrator triggered upon the sensing of the leading edge of the pulse noise.
The device of Japanese publication number 6-71210 has drawbacks mentioned below. The closing of the gate circuit or the signal interpolation is carried out independent of the duration of a pulse noise. Accordingly, there is a possibility that the closing of the gate circuit or the signal interpolation will be carried out during a time excessively longer than the duration of a pulse noise.
In the device of Japanese publication number 6-71210, the signal interpolation is implemented by merely using the output signal of the FM detection circuit which occurs immediately before the gate circuit is closed instead of the current output signal of the FM detection circuit. The signal interpolation of this type tends to cause a significant signal distortion.